This invention relates to coating tin, zinc or steel surfaces with a protective coating that permits acceptable paint adhesion to the surfaces so coated and to the coated tin, zinc and steel articles. More particularly, the present invention provides a protective tin coating by electrolytically coating the tin with a composition having as its essential ingredients amino acids, proteins, amino acids-protein, amino alcohols and mixtures thereof; or inorganic acids and a method of coating the tin surface of the articles and the tin articles so coated; and the coated tin articles which are painted. My invention also provides for a zinc, or plated zinc, or steel coated with a protective coating having a coating composition containing hydroxy benzoic acids; the articles so coated; and the articles coated and painted.
Unpassified zinc will quickly form a thin film of zinc oxide which will prevent the adhesion of paint. Passification will prevent the growth of zinc oxides (see British patent No. 592,072; Wendorff, Z., Zolnierowicz, A.; Ochronaprzad Korozja, 13, 1 (1970); Ostrander, G. W.: Plating, 38 1033 (1951); and British Patent No. 594,699). Typical passification processes use a dichromate or a chromate composition. The compositions are applied by via simple immersion or by electrochemical methods (see Fishlock, D. J.: Product Finishing, 12, 87 (1959). A number of different PH""s, immersion times and temperatures may be used. The use of a chromate or dichromate passification method will generally increase the salt-spray (xe2x80x9cASTM-B117xe2x80x9d testing specification) corrosion resistance of a zinc passified surface by a factor of ten to thirty depending upon the method of passification used (see: Stareck, J. E., Cybulskis, W. S.: Proc. Am. lectroplaters Soc. 34, 235 (1947). As such chromate, or dichromate, compositions are generally considered to produce the most corrosion resistant of films. The hexavalent chromium present in the chromate and/or dichromate compositions is extremely toxic and as such is being banned from use in Europe and many areas of the United States.
The thin natural oxide film on tin surfaces provides a protective barrier and improves paint adhesion. Maintaining this oxide film while preventing a rapid uncontrolled growth to a thick yellow non-protective and non-adhesive layer has always been the goal of tin plate producers. In addition, foods high in sulfur will stain tin surfaces not properly passified.
Previous attempts at passification of tin have centered around the thickening of the natural oxide film with an oxidant while leaving a corrosion resistant film on the surface of the metal to retard further oxide growth and prevent sulfide stains. In 1931 S. R. Mason (Mason, S. R., U.S. Pat. No. 1,827,204) patented an electrolytic process which used chromates to both thicken the oxide film and leave a film of reduced chromic oxides to prevent further oxide growth or the formation of sulfide stains. In 1935 a French patent (Tichauer, French Patent 777,314) detailed a process which used molybdates, an oxidant and various heavy metals to give a passive film on the surface of the metal. In the same year U.S. Pat. No. 2,024,951 described a process which used potassium permanganate to both stabilize the oxide film and reduce sulfide staining. In 1940 U.S. Pat. No. 2,215,165 described an electrolytic process which oxidized and then reduced the tin surface to thicken the oxide film and leave a passive tin surface. In 1943 W. O. Cook and H. E. Romine (U.S. Pat. No. 2,312,076) obtained patents on a process which used dichromates mixed with phosphates to passivate tin surfaces. Since that time all processes have centered around improvements in this basic chromate/dichromate process. Once again, chromate use is being restricted in Europe and the United States and in many cases has been banned from use.
The widely used method for increasing the paint adhesion of steel is to form a film of iron phosphate on the surface of the metal and then xe2x80x9csealxe2x80x9d the phosphated surface with a chromate or dichromate composition (see: Mohler, J. B., Metal Finishing, 69,10,47 (1971) for increased corrosion resistance. Increased restrictions on the use of chromate compositions in the United States and Europe are making this process more and more difficult to use.
My inventions eliminates the need for hexavalent chromium compositions which, due to their extreme toxicity, are being removed from the work place environment. In addition my processes provides the same high level of corrosion resistance, paint adhesion and, in the case of tin, sulfide stain resistance. My processes are less expensive to produce and free of toxic chemicals which require expensive disposal methods for their removal.
I. Tin Surfaces:
I provide a protective coating for tin surfaces such as tin and tin coated steel. The protective coating has as its essential ingredients proteins, amino acids or amino acidxe2x80x94protein compounds and amine alcohols. The process uses electrolysis of various protein salts made by dissolving the proteins in an acid solution, or by dissolving the proteins with a base. The amino acids, amines, amine alcohols or inorganic compounds may be added to the mixture to complex with the proteins and enhance their paint adhesion or protective ability. The lower limit of the concentration of these solutions is purely an economic matter. The lower the concentration of the materials to be deposited, the longer it will take to produce a film of sufficient thickness (about 600 nm) to provide a good paint base and sufficient resistance to oxidation and sulfide staining. Coil coating lines for tin coated steel normally do not allow for more then five seconds of exposure at a current density of about 10 to 25 amps per square foot. This will normally require a concentration of at least 0.5%, depending upon the composition of the mixture in question. The upper limits on the concentration of the solution will be the saturation point of the mixture in question. In theory any PH may be used, but tin dissolves in strongly acid or basic solutions. The most suitable PH range is 2.0 to 12.0 with the preferred PH being 2.5 to 11.0. Temperature is of no concern to the process. The voltage must be above the reduction potential of the protein complexes and sufficient to maintain the required current density. Various other non-interfering materials may be added to the protein solutions to prevent biological attack, act as wetting agents, increase conductivity, improve paint adhesion or to control the PH (buffers) as long as these materials do not act to prevent proper film formation.
In the following Examples 1-11 a tin plated steel surface or a pure tin sheet was cleaned of oils and/or loose dirt with a non-ionic detergent and then made the anode of an electrolytic cell of 12 volts and a current density of about 10 amps per square foot for 30 seconds in a solution of 3.0 grams per liter sodium carbonate to obtain a clean and reactive surface. The surfaces were then rinsed in D. I. (deionized)
water and treated as indicated. The metal strips were 4 inches by 10 inches. Examples 4, 8 and 9 illustrate the outer limits of the specified PH range.